IF and a MUD
There's an online community filled with fans of interactive fiction and the occasional quest. Have you been there yet?

Adventure Games Live
Samuel Stoddard has written a CGI engine for playing adventure games. You select options from menus, though the range of choices is usually more complex than your standard Choose-Your-Own-Adventure.

Adventure on the Web
The original Colossal Cave adventure, playable on the web! The adaption was done by Tom Wu, and is a good one.

Clicking Anastasia
A web game in which you attempt to unearth the Romanov fortune.

Crime Scene
Tom Arriola's site lets you help solve crimes. Though it is more of a "collaborative fiction" site, it's well-done and should appeal to fans of detective IF.

The Explore Adventure Series
A long time ago, Joe Peterson created a simple text adventure system called "Explore", which was written using BASIC and an old TRS-80. Now Joe has ported it to Java, so that you can play them from the comfort of your own browser.

The Hitchhiker's Guide to the Galaxy
Douglas Adams has made his famous Infocom text adventure playable on the web for anyone with a Java-enabled browser.

IAGE Demo
The Internet Adventure Game Engine includes an online demo of the system.

Interactive Fiction Playground
Another repository of z-code text adventures which you can play thanks to ZPlet. This archive is maintained by Curt Siffert. Note that this version of ZPlet allows you to save and restore games.